Dozens of different VTOL aircraft configurations have been proposed and built in the last five decades. See, for example, "An Introduction to V/Stol Airplanes"), Iowa State University Press, 1981. Virtually all of these aircraft rely on some form of thrust modulation of one or more engines at constant RPM or power settings to achieve pitch and roll control during hover flight. By way of example, helicopters vary the pitch of the rotor blade to affect increases or decreases in the lift vector, while tilting the rotor blade using cyclic controls to achieve pitch and roll. Effectively, the helicopter varies the pitch of the rotor in arc segments to obtain this type of pitch and roll thrust vectoring. Rotor speed and rotor blade pitch throughout the arc are utilized to change the applied lift. Certain jet aircraft, such as the Harrier, rely on wing-tip bypass air flow mechanisms to provide varying amounts of thrust at the wing tips. More recently, a number of tilt wing aircraft, such as the Boeing, have been the subject of experimentation incorporating combinations of pivotable wings and thrust generating devices to control changes in pitch, roll and lift.
To date, all vertical takeoff and landing aircraft have been unstable in at least the longitudinal (pitch) and lateral (roll) axes, and in the case of helicopters, also unstable in the yaw axes. As a result, VTOL aircraft are difficult to fly and complex in design. At the present time, there have been several proposals for so-called "multi-post" VTOL aircraft design. Multi-post designs receive their name from the multiple source of thrust which are utilized to provide lift and propulsion. Typically, these designs incorporate stability augmentation through the use of redundant flight control computers, which also add significantly to the complexity of the aircraft design.